Fail Of The Week: Where’s Me Jumper?

Just in case you imagine that those of us who write for Hackaday are among the elite of engineering talent who never put a foot wrong and whose benches see a succession of perfectly executed builds and amazing hacks, let me disabuse you of that notion with an ignominious failure of my own.

I was building an electronic kit, a few weeks ago. It’s a modular design with multiple cards on a backplane, though since in due course you’ll see a review of it here I’ll save you its details until that moment. In my several decades of electronic endeavours I have built many kits, so this one as a through-hole design on the standard 0.1″ pitch should have presented me with no issues at all. Sadly though it didn’t work out that way.

Things started to go wrong towards the end of the build, I noticed that the temperature regulator on my soldering iron had failed at some point during its construction. Most of it had thus been soldered at a worryingly high temperature, so I was faced with a lot of solder joints to go over and rework in case any of them had been rendered dry by the excessive heat.

In due course when I powered my completed kit up, nothing worked. It must have been the extra heat, I thought, so out came the desolder braid and yet again I reworked the whole kit. Still no joy. Firing up my oscilloscope I could see things happening on its clock and data lines so there was hope, but this wasn’t a kit that was responding to therapy. A long conversation with the (very patient) kit manufacturer left me having followed up a selection of avenues, all to no avail. By this time a couple of weeks of on-and-off diagnostics had come and gone, and I was getting desperate. Somehow I’d cooked this thing with my faulty iron, and there was no way to find the culprit.

On our hackspace open night I was sitting with the kit in front of me looking again at its various signals. I took out the multimeter and set about the painstaking task of tracing the continuity of every single one of my reworked joints, convinced that way I’d find the ones that were dry. And I wasn’t disappointed, for between a line on the outer backplane socket and the same one on its neighbours I found no circuit. My culprit at last! Then I found another, and another. An entire bus, disconnected from the rest of the sockets. A horrible suspicion began to form, and I took a closer look at the backplane.

This backplane had its start on earlier versions of this kit as a piece of stripboard. You bought the daughter boards, and supplied your own stripboard. I’d naively assumed that the backplane was simply equivalent to a fancy piece of stripboard, but I was wrong. What I had assumed was space for a socket to take the bus away to another board turned out instead to be space for a set of jumpers which isolated the bus on the final socket. In some applications they could be filled with resistors, all I needed was a set of jumpers and my final board would be able to talk to its neighbours. I fitted a double row of pins and installed the jumpers, and as if by magic the kit worked.

So what can I take away from all this? First of all, the jumpers are there in the schematic in the kit instructions, and it’s clear what they do. So I failed to spot that, due to my having seen the previous stripboard versions of the kit. Secondly, I latched onto my faulty iron as the obvious culprit and didn’t look hard enough for anything else. Confirmation bias, you have claimed another victim!

If I can leave anything for you to take away, it’s that we here at Hackaday have to eat our own dog food when it comes to our failures, and that as I have just discovered it’s all too easy to read the instructions without really having read them at all. The next time any of us have a seemingly insoluble fault whose cause we’re convinced we have a handle on we should all try to take a step back and consider whether we’ve got it right or whether we’re barking up the wrong tree. After all, it could save us all the embarrassment of a public fail like the one described here, and that’s got to be worth the effort!

Jumper image: By Anthrax11 [CC BY-SA 4.0], via Wikimedia Commons.

And of course, my apologies to the Sultans of Ping FC for the title of this piece.

Filed under: Fail of the Week

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